During the 2014 independence campaign, European Union officials previously suggested that Scotland would need to negotiate a new EU membership from outside the bloc if it became an independent member state.

However, Sturgeon yesterday suggested that the timing of the vote was linked to hopes for, at the least, a quick return to the EU.

"If the UK leave the EU without Scotland indicating beforehand, or at least within a short time after it, that we want a different relationship with Europe, we could face a lengthy period outside not just the EU but also the Single Market. That could make the task of negotiating a different future much more difficult," Sturgeon said.

"These considerations lead me to the conclusion that if Scotland is to have a real choice, when the terms of Brexit are known, but before it is too late to choose our own course, then that choice choice should be offered between the autumn of next year, 2018, and the spring of 2019."

Sturgeon will ask the Scottish parliament to approve her plan for a referendum next week.